Since the data transmission between a base station and a terminal in a radio system is usually carried out synchronically, the transmission of the base station has to be synchronized all the time. In TDMA systems, the connection is commonly also circuit-switched. The prior art solutions usually utilize hierarchic or plesiochronous synchronization, in which case the base station receives a synchronization signal from a base station controller and the base station controller, in turn, receives the synchronization signal from a mobile services switching centre, etc. A public telephone system and a radio system are normally synchronized at their highest level with an atomic clock which is as accurate as possible. The synchronization signal, which is a signal having a nominal frequency of about 2 MHz, for example, can be transmitted from the base station controller to the base station through a separate cable or over a radio path. The radio connection to the base station controller is not, however, always possible owing to great distance, signal load or the location of the base station. On the other hand, however, using a separate cable causes several problems. Installing a cable is laborious since an under-ground cable has to be buried in the ground and an overhead cable usually requires poles and supports of its own. When the location of the base station changes even slightly, extensive re-installing procedures will be required. Furthermore, it is also expensive to install a cable.